1. Field of the Invention
This invention relates to the field of information storage systems.
2. Background Art
Mass storage for computer or other information systems is typically provided by magnetic media storage systems such as rigid or flexible disk storage systems. A rotating disk having a magnetic media layer on the surface is accessed by a "read/write" head which is used to store and retrieve information from the disk surface. To store information on a magnetic media disk, flux reversals are induced in the magnetic particles comprising the surface. When a magnetic read/write head is passed over the flux reversals, a signal is induced in the head which can be decoded to provide information.
Typically, data is stored on a magnetic disk in series of concentric "tracks" on the surface of the disk. The read/write head moves back and forth radially on the disk so that it can be selectively positioned over one of the tracks. Once in position over a track, the head remains in place as the track rotates beneath it allowing the head to read or write data on the track.
To effectively read and write data, it is necessary that the position of the tracks with respect to the head be known. In addition to knowing which track a head is over, it is necessary to know where on that particular track the head is positioned. In the prior art, position information is provided through the use of servo patterns. A servo pattern is a permanent pattern written onto a storage disk which can be used to provide position information. The servo pattern is detected by a servo head and, by properly decoding the servo pattern, signifies track position. The servo pattern is typically also written as a series of concentric tracks. In a multi-disk storage environment, one entire side of a storage disk may be dedicated to servo information. A servo head accesses this servo disk to read the position information therein. Since the servo head is in a fixed relationship relative to the read/write heads, the position of the servo head can be used to indicate the position of the read/write heads. In addition to having a dedicated surface for servo information, a "sector" servo pattern may be employed in which pie shaped wedges of servo information are interleaved between sections of data information.
In order to improve the efficiency of data storage in magnetic disk systems, it is desired to maximize the data storage capability. Any disk surface area which is dedicated to servo tracks cannot be used for data tracks. One method of freeing up space for data tracks is to "bury" the servo layer beneath the surface of the data disk itself. One such scheme is described in pending U.S. patent application No. 07/116,109 entitled SERVO PATTERN and assigned to the assignee of the present invention. In that scheme, a plurality of servo lines are formed on a data disk extending from the inner track to the outer track. These lines are employed as part of a buried servo scheme such that the disk surface is dedicated to data storage. The servo pattern is written on each side of the disk so that, if the disk were transparent, the respective servo lines would appear to intersect when viewed from above. Servo heads located on each side of the disk detect servo line crossings. By comparing the time difference between crossings of corresponding tracks on either side of the disk, the radial position of the heads can be determined and servo and data tracks can be defined.
Typically, a separate data and servo head are utilized in the disk drive assembly. In one implementation, the data head and servo head are part of a dual core assembly where one core serves as the data head and the other serves as the servo head. The relationship between these two heads must be known to enable a disk drive to read disks written on other disk drive assemblies. For example, if the distance between the servo head and data head on one dual core assembly is one servo track width greater than the separation of the two heads on a second drive, a one track error will be introduced when a disk written on the first drive is read on the second drive. Servo track spacings are on the order of hundreds of micro-inches so small variations in the servo/data head spacing may result in errors of several servo track widths.
These variations are usually detected and compensated for on power up of the drive assembly. An adaptive calibration procedure is implemented to compare the relative servo and data head positions to a stored nominal value. For example, when the data head is located over the first data track, the servo head may be positioned over servo track "n" when the servo/data head spacing is nominal. When this spacing is off, the servo head is positioned over servo track n.+-.m where m is the number of tracks offset introduced by variations in the servo/data head spacing.
This adaptive calibration scheme is effective only if the absolute position of a servo track is known. Such a requirement implies the use of periodic missing bits in the servo pattern. The reliable detection of pulses in a waveform containing missing bits requires peak detection via a differentiation network. This imposes severe constrains on the type of filter which may be used to remove noise coming from the data head when it is writing. It is often desirable to provide a servo pattern in which peak detection is not required, however, in such a scheme, absolute servo track position is not known.
Therefore, it is not possible to utilize the prior art adaptive calibration scheme above to calibrate the drive and determine spacing between the data heads and the servo head. Such spacing must be known, particularly if a disk written on one drive is desired to be utilized on another drive. Typically, manufacturing tolerances result in differences in the spacing between the data head and servo head on dual head assemblies.
In many prior art disk drives which utilize servo tracks for closed loop track following, the spacing between servo tracks is uniform and is equal to the data track spacing. In these drives, it is only necessary to locate data track #0 as a reference point. In these drives, data track #0 serves as a "reference track" and only one reference point is needed.
In some prior art servo schemes, such as that disclosed is copending patent application number 07/116,109 entitled SERVO PATTERN and assigned to the assignee of the present invention, the servo tracks are nonuniformly spaced. The distance between servo tracks, in inches, continuously decreases when moving from the outer radius of the disk to the inner radius. A second difference is that the location of each servo track is defined solely in the time domain. As a consequence, interpolating between servo tracks is remarkably easy. All that is needed is a clock which is higher in frequency than the frequency of the servo pulses.
In some implementations of a nonuniformly spaced servo pattern, the servo track coordinates of each data track are stored in a lookup table in ROM (Read Only Memory). In such a case the table stored in ROM is only valid for recording heads in which the spacing between the two heads in any particular drive is equal to a nominal value. If not, a nonlinear correction factor must be used to calculate the actual servo track corresponding to each data track for that drive.
Therefore, it is an object of the present invention to provide a method and apparatus for determining the difference from a nominal data head/servo head spacing in a disk drive assembly.
It is another object of the present invention to provide a method and apparatus for calibrating a disk drive assembly so that a disk written on one drive may be accessed by another drive.
It is yet another object of the present invention to provide a method and apparatus for calibrating a disk drive assembly without requiring a "missing bit" servo pattern.
It is still another object of the present invention to provide a method and apparatus for calibrating a disk drive assembly wherein no peak detection of the servo pattern is required.